1. Field
The present specification generally relates to infrared detector devices and, more particularly, to infrared detector devices having a strain-balanced superlattice structure.
2. Technical Background
The nBn device structure generally includes an n-type absorber layer, a barrier layer to block majority carriers, and an n-type contact layer. For the InAs/InAsSb superlattice structure (“SLS”) material, band gap tuning is achieved by tuning the Sb composition and the InAsSb layer thickness, while InAs layer is provided as a strain-balancing layer mostly to balance out the strain of the material grown on the substrate of GaSb. The InAs strain-balancing layer does not significantly contribute to optical absorption and band gap tuning. However, due to the close lattice constant between InAs and GaSb, the required InAs layer thickness to balance out the strain is significant.
On the other hand, the minority carrier hole effective mass is very large; on the order of 2 me in the growth direction for MWIR from k·p simulations. This is because the valence band position of −0.62 eV for InAs (relative to InSb) is lower than the SLS valence band position (for MWIR, −0.4 eV to −0.5 eV) in addition to the fairly thick InAs hole barrier layer. This leads to intrinsically lower absorption and decreased carrier transport in the preferred n-type absorber material than other material systems for infrared detection, such as HgCdTe, InSb, InAs/Ga(In)Sb SLS, and the like. As the cutoff wavelength increases, the required InAs strain-balancing layer thickness increases significantly while the valence band position rises at the same time, leading to dramatically lower optical absorption with increased hole-effective mass. This makes the InAs/InAsSb SLS less practical for longer cutoff wavelengths.